The present invention relates to a belt tensioner for use of a V-belt which is spanned between a crank pulley of an internal combustion engine and an alternator, and more particularly to a belt tensioner which can obtain an optimum tension force applied on a V-belt.
As shown in FIG. 1, an internal combustion engine 3 generally has a crank pulley 21, an alternator 1, and a V-belt 2 spanned between the crank pulley 21 and the alternator 1. The rotation of the crank pulley 21 is transmitted through the V-belt 2 to the alternator 1. If the tension force applied onto the V-belt 2 is greater than an adequate value, this impairs the endurance of bearings employed for the alternator 1. Conversely, if the tension force applied onto the V-belt 2 is less than the adequate value, this causes a slippage of the V-belt 2 and further a decrease in an amount of an electric power generated by the alternator 1. In general, a tension force applied onto the V-belt 2 is adjusted by such a procedure as the alternator 1 is rotated around a bolt 13 mounted on the internal combustion engine 3. A worker sets an optimum tension force while the worker measures the amount of a tension force applied on the V-belt 2. However, it is difficult to easily set an optimum belt tension force. To ease the adjustment operation of a belt tension force, there is proposed a belt tensioner 5 as shown in FIG. 2. (This belt tensioner 5 is disclosed in the Japanese Laying-open Utility Model Publication Sho 55-86144.)
According to the belt tensioner 5 shown in FIG. 2, an optimum belt tension force is obtained by the way such that a bolt 6 only is rotated. The tension force adjustment is made as following procedure. Firstly, a worker loosens the bolt 13 which fixes an alternator 1 to a flange portion 46 of an internal combustion engine 3. Nextly, the worker loosens a nut (not shown in drawings) which fixes the alternator 1 to a bracket 12 in association with a bolt 4. Further, the worker rotates the bolt 6. FIG. 3 illustrates an enlarged cross-sectional view taken along the line III--III in FIG. 2. As shown in FIG. 3, an adjust bolt 8 has a thread engagement with a head portion of a bolt 4 which fixes an arm 9 of the alternator 1 to the belt tensioner 5. When the bolt 6 rotates in the direction D shown in FIG. 3, the adjust bolt 8 rotates with the bolt 6. This rotation moves the arm 9 of the alternator 1 in the direction B shown in FIG. 3. As a result, the alternator 1 rotates around the bolt 13 in the clockwise direction A shown in FIG. 2. Hence, according to the rotation of the alternator 1, the tension force of the V-belt 2 increases. According to the increase in the tension force applied on the V-belt 2, there is generated a force for pulling the adjust bolt 8 in the direction G shown in FIG. 3. When the adjust bolt 8 is pulled in the direction G shown in FIG. 3 against the biasing force of the spring 11 by the stroke greater than the distance corresponding to the height H of a pawl 7, a head portion 10 of the adjust bolt 8 is disengaged from the pawl 7. In this condition, even if the bolt 6 rotates, the rotation of the bolt 6 is not transmitted to the adjust bolt 8. Hence, the V-belt 2 cannot be applied with more tension force. In this circumstances, the force of the spring 11 becomes equal to the tension force of the V-belt 2. In this belt tensioner 5, the spring constant of the spring 11 is designed to have such a value as the tension force applied on the V-belt 2 becomes the appropriate value when the force of the spring 11 becomes equal to the tension force of the V-belt 2. Thus, when the bolt 6 rotates idle, the worker stops the rotation of the bolt 6. The worker fastens the nut (not shown in drawings) which has a thread relation with the bolt 4 and fastens the bolt 13 shown in FIG. 2, thereby fixing the mounted position of the alternator 1. Thus, the tension force of the V-belt 2 is adjusted. The belt tensioner 5 has a simple structure and can easily set the tension force of the V-belt 2 to the optimum value. However, this belt tensioner 5 is not preferable from the stand point that the belt tensioner 5 has the following disadvantages. According to the belt tensioner 5, it performs such an operation as the pawl 7 disengages from the head portion 10 of the adjust bolt 8. Immediately before the pawl 7 disengages from the head portion 10 of the adjust bolt 8, the pawl 7 is engaged with only a small portion of the head portion 10. When the pawl 7 disengages from the head portion 10 of the adjust bolt 8, the pawl 7 strikes against the head portion 10 of the adjust bolt 8 and the edge portions of the pawl 7 and the head portion 10 wear down. While a vehicle is employed, the belt tensioner 5 is repeatedly adjusted by such a way as the pawl 7 is disengaged from the head portion 10 of the adjust bolt 8. Hence, the edge portions of the pawl 7 and the head portion 10 further wear down. This causes the earlier disengagement between the pawl 7 and the head portion 10 of the adjust bolt 8, than the initially set timing. In other words, the more the belt tensioner 5 is adjusted, the weaker the tension force of the V-belt 2 is adjusted.